Statistically ranking Theis compared to other centers discounts the impact that the coach, team and system have on those numbers. If you look at how some people played here vs. other locations, you will generally notice the positive impact Brad Stevens, Celtic culture and other players have had on the players being evaluated. Jordon Crawford was a player of the month (I think) under Brad Stevens and literally did nothing anywhere else. Evan Turner was made useful here and got a huge contract from Portland and stopped producing there. Jae Crowder was unleashed in Boston. IT4 never played better than he did in Boston. Aaron Baynes got better here before taking his talents to Phoenix.
My point is that if Turner is potentially starting off with more raw talent than Theis, who is to say that Brad Stevens will not find better use of that talent? Turner has some fantastic tools and I would bet that he can be better than Theis under the same system and with the same teammates.
Obviously we can all debate whether my comment is true or false and Danny Ainge (if given the opportunity by Hayward) will have the final say as to whether or not he believes it to be true, but in order to make a comparison, the situations need to be equalized.